Putting the "flex" in flexible mobile wireless radios - A wideband continuous-time bandpass sigma-delta ADC for software radios

There are various issues to address in the design of future software radios with focus on analog-to-digital converters (ADCs). In order to cope with different standards such as cellular and WLAN in a flexible radio, ADCs needs to have a variable bandwidth and dynamic range. Sigma-delta modulator architecture ADCs are quite suitable for this task. This modulator allows inherent trade-off between bandwidth and dynamic range and lowers the specifications for the channel/ anti-aliasing filter in front of the ADC. Sigma-delta ADC offers great features for flexibility, adaptability and programmability, high reusability and integration capability. Various design architectures include single stage, cascaded, high-order order, single bit, multibit, switched-capacitor, continuous-time, lowpass, and bandpass sigma-delta modulators. Each architecture offers its merits and drawbacks and is particularly suitable for certain applications. Among them, continuous-time bandpass sigma delta ADC is a promising candidate for high intermediate frequency receivers. Moreover, digitally tuned bandpass sigma-delta modulators are able to change the center frequency and/or the signal bandwidth of the modulator, increasing ADC's flexibility. However, to overcome issues on clock jitter and excess loop delay special design solutions are necessary.